For decades, polymer and plastics manufacturers have used antimony oxide (also referred to as antimony trioxide or ATO) in their formulations to improve fire resistance. In combination with other halogenated flame retardants, it delivers a synergistic effect, significantly improving their efficacy. This makes ATO an easy go-to for meeting fire safety requirements in products such as piping, wiring, cables, household goods, and construction materials.
However in recent years, restricted supplies, volatile pricing, and toxicity concerns about ATO have accelerated the search for alternative fire retardancy additives in polymers.
If you’re involved in the production of polymers and plastics, you’ll want to read our latest technical bulletin, Firebrake® ZB: An alternative to antimony oxide. In this research review, I do a deep dive into the growing body of studies that explore the use of Firebrake ZB—a unique zinc borate—as a partial or complete replacement for ATO in polymer systems.
In this article, I summarize some key insights from those studies. To understand the details of the studies and see the data from their results, download the technical bulletin.
Multifunctional benefits for fire retardancy in polymers
The use of boron as a flame retardant is well established. Borates offer significant benefits during a fire. When polymers treated with zinc borates are exposed to flame, they are slow to ignite, instead producing a char that acts as an insulator to slow the spread of flames while suppressing smoke and toxic fumes.
The demand for more sustainably manufactured polymer products is growing, and you may be looking for flame-retardant additives that help you improve your formulations to meet that demand.
The Phosphorus, Inorganic and Nitrogen Flame Retardants Association (Pinfa)1 defines an ideal flame retardant as one that:
- Isn’t toxic to people, animals, or plants
- Doesn’t migrate out of finished products
- Doesn’t release additional toxins or corrosive gases during a fire
- Doesn’t impede recycling of products
- Is biodegradable or neutral in the environment
These considerations make the research review insights especially timely if you’re looking to enhance your polymer formulations.
Because they are naturally occurring minerals and are effective on their own or in combination with other additives, borates offer a safer alternative for flame retardancy. The research findings consistently highlight Firebrake ZB as a cost-effective, high-performance ATO alternative that can help you:
- Reduce dependency on restricted ATO sources
- Reduce vulnerability to ATO price fluctuations
- Achieve reliable fire protection across a wide variety of polymer applications
And as an added bonus, the Firebrake family of zinc borates have high thermal stability, supporting consistency in processing and in product performance.
Validating Firebrake ZB as an antimony oxide alternative
The studies reviewed in the new technical bulletin examined Firebrake ZB performance across a wide range of polymers under a variety of conditions and through a range of tests, depending on the polymer type and application, including:
- Oxygen index
- Smoke emission and NBS smoke chamber studies
- Cone calorimeter
- Mass loss calorimeter
- Limited oxygen index (LOI)
- Electrical property studies
Depending on the polymer application, some studies also measured conformance to UL94 or Federal Aviation Administration fire testing standards.
Some notable results:
- Polyvinyl chloride (PVC) formulations: When Firebrake ZB partially replaces ATO, the zinc borate lowers heat release while maintaining or improving fire retardancy. The most notable improvement is zinc borates’ smoke suppression, which is key for safety in PVC applications.
- Epoxies: Firebrake ZB outperforms ATO when combined with certain halogens (eg alicyclic compounds), improving both fire resistance and smoke suppression. With aromatic halogens, Firebrake ZB is slightly less effective but still offers valuable partial replacement options for reducing smoke.
- Halogenated polyethylene: Partial replacement of ATO with Firebrake ZB maintains excellent fire retardancy, achieving a UL94 V-0 rating, which means that flame self-extinguishes within 10 seconds with no dripping.
- Nylon: Firebrake ZB works synergistically with ATO, maintaining fire resistance at a UL94 V-0 rating while significantly improving properties such as electrical qualities, thermal stability, color stability, melt viscosity stability—even corrosion resistance of the processing equipment.
- Acrylonitrile butadiene styrene (ABS) formulations: Up to 75% of ATO can be replaced with Firebrake ZB while still maintaining a V-0 fire resistance rating and improving smoke suppression. This replacement also improved tensile mechanical properties, making it a safer and stronger option for ABS applications.
- High-impact polystyrene (HIPS): Firebrake ZB can replace up to 75% of ATO and still maintain a V-0 fire resistance rating. It also reduces afterglow, which is high when ATO is used alone.
- Polyester: Firebrake ZB works well in polyester resins with alicyclic halogen sources, maintaining fire resistance and translucency, which is not achievable with ATO. However, it is less effective in polyesters with aromatic halogens.
- Halogenated elastomers, such as styrene-butadiene rubber (SBR) or neoprene, a 30-50% replacement of ATO with Firebrake ZB achieves similar or better fire performance, opening the door for a potential complete replacement.
- Urethane: When added to ATO, Firebrake ZB enabled a polyurethane-backed carpet to pass stringent aviation fire safety tests, highlighting its value in critical applications like aircraft interiors.
Finding better ways to provide fire retardancy for polymers
When you’re seeking to improve your products, processes, fire-retardancy compliance, and your bottom line, it helps to know you have options.
The findings in the reviewed studies highlight Firebrake ZB as a viable alternative to ATO. In addition, our own fire retardancy researchers are always adding to our body of knowledge—and developing innovative opportunities for better flame retardancy solutions.
When you have questions, connect with the U.S. Borax technical team. You’ll tap into decades of scientific research and practical experience to help optimize your formulation.
Resources
References
1 About Pinfa: Criteria for the ideal flame retardant